Lead storage batteries are widely used in various applications, such as in-car applications for driving automobiles or for use as power supply to various types of electric loads, industrial applications for use as backup power supplies for commercial power supplies, and electric vehicle applications for use as main power supplies for golf carts, forklifts, and the like. Lead storage batteries are commonly used, such as six batteries connected in series for 12 V, and batteries configured for 24 V, 36 V, 48 V, 60 V, 72 V, and the like as multiples of 12 V.
Lead storage batteries are low in price as compared with storage batteries such as lithium ion storage batteries and nickel-metal-hydride storage batteries, but inferior in charge/discharge cycle characteristics as compared with the storage batteries. In particular, lead storage batteries have charge/discharge cycle characteristics degraded significantly on reaching an overcharge state or an overdischarge state. For example, when a lead storage battery reaches an overcharge state, gas generation and a decrease in amount of electrolytic solution, corrosion of a current collector, and loss of an active material due to electrolysis of sulfuric acid as an electrolytic solution will be caused, thereby degrading charge/discharge cycle characteristics. Alternatively, when a lead storage battery reaches an overdischarge state, positive and negative electrode surfaces covered with a lead sulfate which is a reaction product and an insulator will inhibit smooth charge/discharge reactions, thus degrading charge/discharge cycle characteristics.
Therefore, in particular, when only a lead storage battery is used as a storage battery for an in-car application or an electric vehicle, there is concern about early deterioration of the lead storage battery. When this concern is addressed by simply replacing the lead storage battery with a storage battery which is superior in charge/discharge cycle characteristics to lead storage batteries such as lithium ion storage batteries and nickel-metal-hydride storage batteries, an increase in the price of the storage battery will be caused.
Therefore, Patent Document 1 proposes a power storage system where an inexpensive lead storage battery and a high-performance storage battery which is superior in charge/discharge cycle characteristics to the lead storage battery are connected in parallel.
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-131134